1. Field of the Invention
The present invention relates to an improvement in the chlorine bleaching process and apparatus and in particular, relates to an improved method and apparatus for decreasing the charge of alkali required in the bleaching extraction stage thereof.
2. Description of the Prior Art
The recycling of some of the extraction effluent during chlorination pulp bleaching is a common practice, either to conserve heat values or to displace the acidic chlorination liquor. The recycled extraction stage liquor is added to the chlorinated pulp as it leaves the chlorination stage washer via the washer showers and this recycled liquor is carried with the pulp back into the extraction stage (Histed, J. A. and Nelson, Jr., G. G. in "The Bleaching of Pulp" , R. P. Singh, Ed., 3rd Edition, TAPPI Press, Atlanta, 1979, p. 393; Histed, J. A. and Nicolle, F. M. A., Pulp Paper Mag. Can. 74 (5) T171 (1974); Wartiovaara, I. Paperi ja Puu 62 (5) 319 (1980); and Wartiovaara, I., Pulp Paper Can. 81 (7) T167 (1980)).
It is also known in the prior art (Burkart, L. F., Paper Trade J. 156 (2) 33 (1972) and Azad, A. M. and Burkart, L. F., Tappi 59 (4) (1976)) to use recycled effluent fortified with sodium hydroxide as the extracting liquor for the first extraction stage. This is found to decrease the amount of sodium hydroxide required in the extraction stage by about 16%.
Limerick (Re 23,868) discloses (col. 3, lines 42-51) a process in which a portion of the alkaline effluent from an extraction stage washer which follows an extraction stage "steeping tower" (in which the chlorine values of a chlorinated pulp are extracted from the pulp with caustic previously mixed therewith) is used to dilute the stock going to that washer by recycling to the second of a pair of post-chlorination pre-caustic extraction washers, where some is used as shower water and some is used to dilute the pulp leaving that washer. However, this method of recycling the alkaline extraction stage effluent does not result in any significant savings in the caustic required in the extraction stage because the acid values which react with the recycled effluent generate like amounts of caustic-consuming sodium bicarbonate and CO.sub.2, which are carried with the pulp and react with the alkali added to the pulp directly after the post-chlorination, pre-caustic extraction washers.